The multimedia broadcast/multicast service (MBMS) is a service that provides broadcasting and multicasting services to mobile phones. The 3rd Generation Partnership Project (3GPP), which is a standardization group for the Wideband Code Division Multiple Access (WCDMA) system, is working on the standardization of the MBMS.
In the 3GPP WCDMA system, both transmitting part and receiving part are composed of an application layer, a plurality of middle layers, and a physical layer. In the transmitting part, data signals are transmitted and processed in a sequence of the application layer, the middle layers, and the physical layer. In the receiving part, data signals are transmitted and processed in a sequence of the physical layer, the middle layers, and the application layer.
Signals that pass through a wireless channel, such as a mobile phone, may have diverse errors due to fading and interference signals in the wireless channel. Conventional WCDMA systems use turbo codes of the physical layer as forward error correction codes to correct errors in the wireless channel, perform encoding in a transmitter of the physical layer, and perform decoding in a receiver.
After the encoding is performed in the transmitter, a Cyclic Redundancy Check (CRC) bit is added. The receiver decodes the signals received through the wireless channel, and performs the CRC checking with a hard bit, which is a result of decoding, to check whether there is an error in the decoding result.
A data transmission method based on a 3GPP Transport Specification (TS) 25.322 Radio Link Control (RLC), which is one of the conventional WCDMA specifications, is composed of an unacknowledged mode (UM), an acknowledged mode (AM), and a transparent mode (TM). The RLC Protocol specification specifies that data with a CRC error is to be discarded in the unacknowledged mode and the acknowledged mode. Therefore, an MBMS receiving system which uses the unacknowledged mode and receives an MBMS service discards the PDU with a CRC error according to the RLC protocol specification.
When a conventional WCDMA system Release-6 receives another service which is not an MBMS service, it performs power control, adaptive modulation and coding (AMC) or hybrid automatic repeat request (HARQ) to overcome the error of the wireless channel.
In short, in case of a real-time service, power control or soft handover is used, and the error correction code for correcting an error is used only in the physical layer. In case of a non-real time service, AMC or HARQ is used and the error correction code for correcting an error is used only in the physical layer.
However, since the MBMS service provides the same signals to a plurality of users, the power controlling method, AMC and HARQ cannot be used. The MBMS system has an enhanced error occurrence probability for those with a poor wireless channel environment. Thus, the MBMS system requires another error correction technology, which is different from the conventional error correction technology described above.
In response to the demand, a technology of encoding data with raptor codes and decoding the data in the application layer of an MBMS terminal is adopted as a standard technology for error correction of the MBMS systems in June 2005.
FIG. 1 shows an exemplary view of an MBMS protocol. As shown in FIG. 1, MBMS services are categorized into a streaming service and a download service.
In case of the streaming service, the RLC upper layer is composed of a packet data convergence protocol (PDCP), an Internet Protocol (IP), a User Datagram Protocol (UDP), and a Real Time transport Protocol (RTP).
In case of the download service, the RLC upper layer is composed of a PDCP, an IP, a UDP, and a File Delivery over Unidirectional Transport (FLUTE) protocol.
In the streaming service, raptor encoding and decoding are carried out in the RTP layer, while they are performed in the FLUTE layer in the download service. The position of the raptor codes in the application layer can be detected by referring to FIG. 1.
An MBMS bearer is composed of a PDCP, an RLC, a medium access control (MAC), and a physical layer. The PDCP performs a role of compressing a header, and it is specified to be optional in the conventional standard specification. Herein, “optional” means that the element is not essential and it can be realized selectively.
A communication system such as the MBMS system includes a plurality of protocols. An input data unit is referred to as a service data unit (SDU) and an output data unit is referred to as a protocol data unit (PDU) in all protocols.
Protocols of higher layers than the layer of a certain protocol are called protocols of upper layers. In other words, in the respect of the physical layer, the RLC layer and the application layer are upper layers, and in the respect of the RLC layer, the application layer is its upper layer.
The data encoded in the application layer are processed in the middle layers based on the protocols of the middle layers, and inputted to the RLC layer, which is one of the middle layers. An SDU may be segmented into a plurality of PDUs or concatenated with a preceding SDU or the following SDU to thereby form a PDU, or it becomes a PDU without division or concatenation.
A PDU of the RLC layer is inputted to the physical layer based on an MAC protocol. Herein, the input unit is referred to as a transport block. In the physical layer, a CRC bit is added to the transport block, and encoding is carried out with an error correction code. Encoded data are transmitted to the receiving part through a wireless channel.
FIGS. 2 and 3 show block diagrams of an MBMS system to which the present invention is applied. As illustrated in FIGS. 2, presented is an example of a Long-Term Evolution (LTE) Universal Mobile Telecommunications System (UMTS). Although the present embodiment shows a structure where BM-SC 100, which is proprietary to the MBMS service, and an access gateway (aGW) 300 supporting unicast, which is a non-MBMS service, are separated, it is possible to realize the two gateways integrated. MBMS data pass the MBMS gateway 200, and they are transmitted to the terminal 500 though an enhancement node B (eNB) 400, which is a sort of a base station. The physical layer of the terminal 500 receives the data through the wireless channel, performs decoding, and checks a CRC error.
According to the 3GPP Release-6 TS25.322 Specification, the RLC layer of the transmitter creates PDUs by concatenating or segmenting SDUs.
The evolved node B 400, which is the base station, determines the size of the PDUs during the MBMS service. When SDUs are inputted to the RLC layer and an SDU is larger than a PDU, the SDU is segmented to have a size equal to or smaller than the PDU.
If there is an SDU which has arrived before but remains without transmission, it is concatenated with the current SDU to thereby create a PDU.
FIG. 4 is a view showing an example that SDUs are segmented or concatenated to form PDUs. Referring to FIG. 4, the first PDU is created by segmenting an SDU, and the second PDU is created by concatenating and segmenting SDUs.
The PDUs acquire CRC bits added thereto in the physical layer after passing through the MAC layer, go through turbo encoding, and are transmitted to the terminal, which is an MBMS receiving system through a wireless channel.
The physical layer of the terminal performs channel decoding, performs cyclic redundancy checking (CRC) to determine whether there is an error in a packet, and transmits the CRC check result and packets except CRC bits to the RLC layer through the MAC layer.
According to the 3GPP Release-6 Specification (TS25.322 RLC), when the SDUs are combined in the RLC layer of the terminal, PDUs with a CRC error are discarded, and all the SDU included in the discarded PDUs are supposed not to be transmitted to the upper layers, regardless of whether the error has occurred in the SDUs.
To be specific, when a CRC error occurs in the second PDU in FIG. 4, all of the second, third, and fourth SDUs are discarded according to the current RLC specification. Even if an error has occurred only in the third SDU, the second and fourth SDUs are discarded together.
Therefore, the conventional error correction method used in the MBMS terminals has a shortcoming that SDUs without an error therein are discarded and thus radio resources are wasted.
In the conventional 3GPP WCDMA system, the physical layer of the receiver performs error correction with a turbo decoder and then performs a CRC error checking. The RLC layer of the receiver recovers SDUs, which are data to be delivered to the upper layer, from the PDUs. Herein, the SDUs are recovered from PDUs without a CRC error. When a PDU with a CRC error is received, another PDU using an SDU part of which is included in the PDU with a CRC error is discarded, too. Eventually, the error correction technology used in the conventional MBMS system brings about an effect that more errors occur than they actually do in the physical layer, and thus induces an error occurrence environment deteriorated more than it actually is.
Meanwhile, according to the 3GPP Release-6 RLC Specification, the application layer reports an error in the RLC layer on the basis of SDU, and corrects the error of a bursty error channel where errors occur continuously. The error correction codes of an erasure channel may show better performance in a random error channel than in the bursty error channel. The smaller the erasure quantity is, the more excellent the performance becomes. Therefore, it is possible to expect improvement in the error correction efficiency by making the conventional MBMS erasure channel environment random and reducing the quantity of erasure.